As formats for compression of audio signals, nonreversible compression formats such as MP3 (MPEG Audio Layer-3), WMA (Windows Media Audio, registered trademark), and AAC (Advanced Audio Coding) are known. In the nonreversible compression formats, high compression rates are achieved by drastically cutting high frequency components that are near or exceed the upper limit of the audible range. At the time when this type of technique was developed, it was thought that auditory sound quality degradation does not occur even when high frequency components are drastically cut. However, in recent years, a thought that drastically cutting high frequency components slightly changes sound quality and degrades auditory sound quality is becoming the mainstream. Therefore, high frequency interpolation devices that improve sound quality by performing high frequency interpolation on the nonreversibly compressed audio signals have been proposed. Specific configurations of this type of high frequency interpolation devices are disclosed for example in Japanese Patent Provisional Publication No. 2007-25480A (hereinafter, Patent Document 1) and in Re-publication of Japanese Patent Application No. 2007-534478 (hereinafter, Patent Document 2).
A high frequency interpolation device disclosed in Patent Document 1 calculates a real part and an imaginary part of a signal obtained by analyzing an audio signal (raw signal), forms an envelope component of the raw signal using the calculated real part and imaginary part, and extracts a high-harmonic component of the formed envelope component. The high frequency interpolation device disclosed in Patent Document 1 performs the high frequency interpolation on the raw signal by synthesizing the extracted high-harmonic component with the raw signal.
A high frequency interpolation device disclosed in Patent Document 2 inverses a spectrum of an audio signal, up-samples the signal of which the spectrum is inverted, and extracts an extension band component of which a lower frequency end is almost the same as a high frequency range of the baseband signal from the up-sampled signal. The high frequency interpolation device disclosed in Patent Document 2 performs the high frequency interpolation of the baseband signal by synthesizing the extracted extension band component with the baseband signal.